1. Field of the Invention
This invention relates to alarm and control techniques for multiplex systems and in particular for systems which use time division multiplexing for the transmission of signaling information.
2. Description of the Prior Art
Alarm and control systems for frequency division multiplex (FDM) and pulse code modulation (PCM) carrier system are well known in the art. An example of an alarm and test technique for FDM system is disclosed in U.S. Pat. 2,986,610. In this arrangement, the signal transmission via the transmission path is monitored by use of a 96 kHz pilot tone which is transmitted along with the double sideband FDM channel signals. At the receiving terminal the 96 kHz tone is rectified and the amplitude of the rectified voltage is compared with a reference. A transmission failure is identified when the rectified voltage is above or below predetermined maximum or minimum values. Action is then taken to notify the terminal at the other end and to subsequently drop off and then busy out the trunks to prevent a subscriber from being hung up on an inoperative channel. At a fixed time interval following the busy out, the transmission path is tested to determine if the transmission anomaly has self-cleared. If it has, the restoral process is initiated. If not, the terminals lock out in alarm condition.
A similar alarm system is disclosed in U.S. Pat. 3,112,370. In this arrangement, the system employs PCM and use is made of the least significant message digit space of the channel code and the digit space, per channel, which has been added for signaling purposes, to transmit the alarm information between terminals. This is done by forcing a specific binary condition in the least significant message digit, a condition that would be abnormal to the condition that would occur during normal operation. In contrast to the FDM technique, which relies upon the deviation of the rectified amplitude from a predetermined range, a transmission anomaly is recognized, in the PCM system, by an out-of-frame condition, which cannot be corrected within a predetermined time.